Altitude sickness: why thin air affects you and how to prevent it

At high altitude the air still contains about 21% oxygen, but the air pressure is lower, so every breath delivers fewer oxygen molecules. At 2,500 metres you get roughly three quarters of the oxygen per breath you would at sea level, and that is about the height above which altitude sickness becomes a real risk. The altitude oxygen calculator shows the numbers for any height; this guide explains what they mean for your body and how to stay safe.

Same oxygen percentage, thinner air

A common misunderstanding is that high mountains have less oxygen in the air. The proportion is the same, about 20.9%, from sea level to the top of Everest. What changes is pressure: there is less air above you pressing down, so each lungful is less dense. What your body responds to is the oxygen partial pressure, which is 20.9% of the total pressure. As it falls, less oxygen moves from your lungs into your blood. The calculator estimates air pressure with the standard-atmosphere formula and expresses the result as an effective oxygen figure: the percentage of oxygen at sea level that would give the same partial pressure, which is how altitude-training tents describe their settings.

How thin is the air?

Approximate values from the standard atmosphere:

• Denver, 1,609 m: about 82% of sea-level pressure, like breathing about 17% oxygen at sea level.
• An airliner cabin, pressurised to about 2,400 m: about 75%, like breathing 15.6%.
• La Paz, 3,640 m: about 64%, like breathing 13.4%.
• Everest Base Camp, 5,364 m: about half, like breathing 10.6%.
• Everest summit, 8,849 m: about a third, like breathing 6.5%.

Real pressure also varies with weather and latitude, and how people respond varies even more: being fit does not protect you, and someone who was fine on one trip can be ill on the next. Water boils at a lower temperature as pressure falls, which is why cooking pasta or rice takes longer at altitude; the boiling point calculator shows by how much.

Symptoms of altitude sickness

According to the NHS, symptoms usually start 6 to 10 hours after arriving at a high altitude. The main ones are:

• a headache;
• loss of appetite, feeling sick or being sick;
• tiredness or exhaustion;
• dizziness;
• difficulty sleeping.

It can feel a lot like a hangover. If you feel unwell at altitude, assume it is altitude sickness until shown otherwise and tell someone you are travelling with. Rest at the same height and do not go higher; painkillers or anti-sickness medicine can ease the symptoms. Symptoms usually improve in one to three days. If they get worse or have not improved after a day, go down, ideally 300 to 1,000 metres.

How to reduce the risk

The key is a slow ascent, which gives your body time to acclimatise by breathing faster and deeper and, over days and weeks, carrying more oxygen in the blood. NHS advice includes:

• spend a few days below 2,500 m before going higher, and avoid going from below 1,200 m to above 3,500 m in a single day;
• above 3,000 m, try not to sleep more than 500 m higher than the night before, and take a rest day every three to four days;
• if you fly directly to a high place, rest for a day before going any higher;
• drink enough water and avoid alcohol;
• see a GP or travel clinic before the trip if you have had altitude sickness before or must ascend quickly, as they may prescribe medicine to help prevent it.

Walking uphill at altitude is slower than at home, which matters for planning a day; the hiking time calculator estimates time from distance and climb, so add a generous margin on top. Dehydration causes headaches and tiredness too, and Signs of dehydration, and what to do about them is worth reading before a trek.

Warning signs: descend and get help

Altitude sickness can progress to two life-threatening conditions: fluid on the lungs (high altitude pulmonary oedema) and swelling of the brain (high altitude cerebral oedema). Get medical help immediately, and go down 300 to 1,000 metres straight away if you can, if you or someone with you:

• is confused or has problems with balance or coordination;
• is short of breath even when resting;
• has a cough, or is coughing up frothy or bloody spit;
• has blue or grey lips, skin, tongue or nails;
• is very sleepy or difficult to wake, or is seeing or hearing things that are not there.

Do not leave someone with these symptoms alone. The calculator tells you how thin the air is, not how your body will cope with it; this guide is general information, not medical advice.